Energy release using tunable reactive materials

ABSTRACT

A reactive material stack with tunable ignition temperatures is provided by inserting a barrier layer between layers of reactive materials. The barrier layer prevents the interdiffusion of the reactive materials, thus a reaction between reactive materials only occurs at an elevated ignition temperature when a certain energy threshold is reached.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.:N00014-12-C-0472 awarded by the Office of Navy Research. The Governmenthas certain rights in this invention.

BACKGROUND

The present application relates to reactive material stacks, and moreparticularly to reactive material stacks with tunable ignitiontemperatures.

Reactive materials are a class of materials which can react to generateheat through a spontaneously exothermic reaction without producinggaseous products or generating a large pressure wave. Reactive materialsare thus useful in a wide variety of applications requiring generationof intense, controlled amount of heat, including bonding, melting andmicroelectronics where the release of energy needed can be triggered byexternal ignition with, or without, a source of oxygen. For certainapplications, it may be important that the energy stored in the reactivematerials is not released until needed. For example and when employed aserasure elements to induce phase transformation of phase changematerials of phase change memory (PCM) cells in an integrated circuitchip, the reactive materials need to be benign during the backend-of-line fabrication process (which typically requires annealing thechip at a temperature up to 400° C.) and normal chip operations, but canbe ignited quickly when a triggering event occurs, e.g., when the chipis compromised (e.g., lost or stolen) and a possibility of a securitybreach could occur.

SUMMARY

The present application provides reactive material stacks with tunableignition temperatures. By separating alternating layers of reactivematerials from one another with a barrier layer, the interdiffusion ofmetal elements of the reactive materials is prevented. The reactivematerial stacks thus remain unreacted until a high energy threshold isreached.

In one aspect of the present application, a reactive material stack isprovided. The reactive material stack includes alternating layers of afirst reactive material and a second reactive material and a barrierlayer located between the layers of the first reactive material and thesecond reactive material, wherein the barrier layer comprises atransition metal, an oxide thereof, a nitride thereof, aluminum oxide(Al_(x)O_(y)) or a combination thereof.

In another aspect of the present application, a method of forming areactive material stack is provided.

In one embodiment, the method includes forming a layer of a firstreactive material over a substrate, forming a barrier layer over thelayer of the first reactive material, forming a layer of a secondreactive material over the barrier layer, forming another barrier layerover the layer of the second reactive material and repeating the formingof the layer of the first reactive material, the forming of the barrierlayer, the forming of the second reactive material, and the forming ofthe another barrier layer to provide a desired thickness for thereactive material stack.

In another embodiment, the method includes forming a layer of a firstreactive material over a substrate, forming a layer of a second reactivematerial over the layer of the first reactive material, forming abarrier layer over the layer of the second reactive material, andrepeating the forming of the layer of the first reactive material, theforming of the second reactive material, and the forming of the barrierlayer to provide a desired thickness for the reactive material stack.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an exemplary reactive material stackthat can be employed in an embodiment of the present application.

FIG. 2 is a cross-sectional view illustrating a barrier layer stack thatcan be employed in the exemplary reactive material stack of the presentapplication.

FIG. 3 is a cross-sectional view of another exemplary reactive materialstack that can be employed in another embodiment of the presentapplication.

FIG. 4A shows a graph of sheet resistance versus temperature for aconventional reactive material stack including a bilayer of Al/Ni formedover a SiO₂ coated substrate.

FIG. 4B shows a X-ray diffraction (XRD) profile of the conventionalreactive material stack.

FIG. 5A shows a graph of sheet resistance versus temperature for a firstexemplary reactive material stack that includes a single barrier layersandwiched between an Al layer and a Ni layer according to a firstexample of the present application.

FIG. 5B shows a XRD profile of the first exemplary reactive materialstack.

FIG. 6A shows a graph of sheet resistance versus temperature for asecond exemplary reactive material stack that includes a barrier layerstack sandwiched between an Al layer and a Ni layer according to asecond example of the present application.

FIG. 6B shows a XRD profile of the second exemplary reactive materialstack.

FIG. 6C shows a graph of sheet resistance versus heating time for thesecond exemplary reactive material stack.

FIG. 7 is a bar graph showing effects of barriers layers on ignitiontemperatures of a Ni—Al reactive material pair.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

Referring to FIG. 1, there is illustrated a reactive material stack 8that can be employed in an embodiment of the present application. Thereactive material stack 8 includes alternating layers of a firstreactive material 10 and a second reactive material 20, and a barrierlayer 30 sandwiched between each layer of the first reactive material 10and the second reactive material 20. The reactive material stack 8typically contains tens to about one hundred of these layers and has atotal thickness from 0.5 μm to 10 μm, although greater or lesserthicknesses may be contemplated.

The reactive material stack 8 can be formed over a substrate (notshown). The substrate can be a semiconductor substrate, a dielectricsubstrate, a conductive material substrate, or a combination thereof. Inone embodiment, the substrate can include a bulk semiconductorsubstrate, a semiconductor-on-insulator (SOI) substrate or a III-Vsemiconductor substrate as known in the art. The substrate may alsoinclude metal lines and/or metal via structures embedded within at leastone dielectric material layer.

The first reactive material and the second reactive material areselected to react with one another in an exothermic reaction uponignition. In one embodiment, such exothermic reaction producessufficient heat to cause the alteration to the memory state of phasechange memory (PCM) cells in integrated circuits. Exemplary sets of thefirst reactive material and second reactive material include, but arenot limited to, Ni/Al, Al/Pd, Cu/Pd, Nb/Si and Ti/Al. Additionalexemplary sets of the first and second reactive materials that may beused in embodiments of the present application are described in “ASurvey of Combustible Metals, Thermites, and Intermetallics forPyrotechnic Applications”, by Fischer et al., 32nd AIAA/ASME/SAE/ASEEJoint Propulsion Conference, Lake Buena Vista, Fla., 1996, thedisclosure of which is hereby incorporated by reference in its entirety.

The reaction of the first and second reactive materials may be ignitedby a mechanical stress, an electric spark, a laser pulse, or othersimilar energy ignition sources. Upon ignition, metal elements of thefirst reactive material and second reactive material intermix due toatomic diffusion to form an alloy, intermetallic or a composite of thefirst reactive material and the second reactive material. The change inchemical bonding, caused by interdiffusion and compound formation,generates heat in an exothermic chemical reaction.

The layers of the first and second reactive materials 10, 20 may beformed using conventional film deposition techniques such as, forexample, physical vapor deposition (PVD) or chemical vapor deposition(CVD), atomic layer deposition (ALD), electroplating and spin-on(sol-gel) processing. The thickness of each layer of the first reactivematerial 10 and the second reactive material 20 may range from 1 nm to200 nm, although lesser or greater thicknesses can also be employed. Thethickness of the layers may be a constant or some layers may have adifferent thickness than others.

Each barrier layer 30 acts as a diffusion barrier to reduceinterdiffusion of the first and second reactive materials, thuspreventing the reactions from taking place until a triggering eventdesignated to initiate the reaction occurs. Each barrier layer 30 mayinclude transition metals selected from Group IVB or VB of the PeriodTable of Elements, oxides of these transition meals, nitrides of thesetransition meals, aluminum oxide (Al_(x)O_(y) with x from 1 to 2 and yfrom 1 to 3) or combinations thereof. Exemplary transition metalsinclude, but are not limited to, Ti, Zr, Hf, V, Nb and Ta. Each barrierlayer 30 may be formed of a single layer structure or a multilayer stack(as shown in FIG. 2). In one embodiment, each barrier layer 30 includesa single layer of Ta. In another embodiment, each barrier layer 30includes a stack selected from the group consisting of Ta/Ta_(x)O_(y),Al_(x)O_(y)/Ta/Ta_(x)O_(y) or Al_(x)O_(y)/Ta/Ta_(x)O_(y)/Ta/Ta_(x)O_(y).For example and as shown in FIG. 2, each barrier layer 30 includes afive-layer stack of Al_(x)O_(y) (labeled as 32 in the drawing) andalternating layers of Ta (labeled as 34 in the drawing) and Ta_(x)O_(y)(labeled as 36 in the drawing) with x from 1 to 3 and y from 1 to 5. Itshould be noted that the number of alternating layers in the barrierlayer stack is not limited to four layers as shown in FIG. 2, othernumbers of alternating layers can also be employed in the barrier layerstack. The thickness of each barrier layer 30 may be from 1 nm to 20 nm,although lesser and greater thicknesses can also be employed.

The barrier layers 30 may be formed, for example, by PVD, CVD, ALD,electroplating or spin-on (sol-gel) processing. In one embodiment andwhen transition metal oxides or metal nitrides are employed in thebarrier layer 30, the transition metal oxide layer or the transitionmetal nitride layer may be formed by first forming a transition metallayer and converting a surface portion of the transition metal layer bythermal nitridation and/or thermal oxidation.

Referring to FIG. 3, there is illustrated another reactive materialstack 8′ that can be employed in another embodiment of the presentapplication. The reactive material stack 8′ includes alternating layersof a first reactive material 10 and a second reactive material 20, and abarrier layer 30 sandwiched between each pair of the layer of the firstreactive material 10 and the layer of the second reactive material 20.Each layer is composed of the same material and can be formed by thesame method as described above in FIG. 1.

The energy required to initiate the exothermic reaction is directlyrelated to the physical properties, e.g., thickness and the compositionof each barrier layer 30. To illustrate the effects of the barrier layer30 on the ignition temperatures of the reactive material stack 8 of thepresent application, a barrier layer or a barrier layer stack of thepresent application is introduced between an Al layer and a Ni layer. Ina first example and when a single barrier layer is employed, a firstexemplary reactive material stack of the present application includes,from bottom to top, 20 nm Al/10 nm Ta/10 nm Ni formed over a SiO₂ coatedSi substrate. In a second example and when a barrier layer stack isemployed, a second exemplary reactive material stack includes, frombottom to top, 20 nm Al/Al_(x)O_(y)/5 nm Ta/Ta_(x)O_(y)/5 nmTa/Ta_(x)O_(y)/10 nm Ni formed over a SiO₂ coated Si substrate. Theoxide layers in the second example were formed by exposing the structureto an air break after deposition of each metal layer. The ignitiontemperatures obtained from the first and second exemplary reactivematerial stacks are compared with a conventional reactive material stackcomposed a bilayer of 20 nm Al and 10 nm Ni formed over a SiO₂ coated Sisubstrate.

FIG. 4A shows a sheet resistance of the conventional reactive materialstack as a function of temperature and FIG. 4B shows a X-ray diffraction(XRD) profile of the conventional reactive material stack as a functionof temperature at a heating rate of 3° C./s in a helium ambient. Asshown in FIG. 4A, the sheet resistance initially increases linearly withincreasing of temperature but deviates from linearity at about 260° C.,indicating that at about 260° C. the reaction between Al and Ni proceedsto form an Al₃Ni₂ alloy. The phase change at about 260° C. is alsoevidenced in the XRD profile. As shown in FIG. 4B, phases of Al and Nidisappear while a new Al₃Ni₂ phase appears after heating to 260° C.Thus, both sheet resistance and XRD measurements indicate that atemperature of 260° C. at a ramp rate of 3° C./scan trigger the reactionof Al and Ni.

FIG. 5A shows a sheet resistance of the first exemplary reactivematerial stack of the present application as a function of temperatureand FIG. 5B shows a XRD profile of the first exemplary reactive materialstack as a function of temperature at a heating rate of 3° C./s in ahelium ambient. As shown in FIG. 5A, the sheet resistance initiallyincreases linearly with increasing of temperature but deviates fromlinearity at about 400° C., indicating that at about 400° C. thereaction between Al and Ni proceeds to form an Al₃Ni₂ alloy. The phasechange at 400° C. is also evidenced in the XRD profile. As shown in FIG.5B, phases of Al and Ni disappear while a new Al₃Ni₂ phase appears afterheating to 400° C. This means that a reaction temperature of 260° C. isnot sufficient to trigger the reaction of Al and Ni when a Ta barrierlayer is present therebetween, but rather a temperature above 400° C. isneeded. Thus, by introducing a 10 nm Ta barrier layer between the Allayer and Ni layer, the reaction temperature for Al and Ni couples canbe increased to 400° C.

FIG. 6A shows a sheet resistance of the second exemplary reactivematerial stack of the present application as a function of temperatureand FIG. 6B shows a XRD profile of the second exemplary reactivematerial stack as a function of temperature at a heating rate of 3° C./sin a helium ambient. As shown in FIG. 6A, the sheet resistance initiallyincreases linearly with increasing of temperature, but deviates fromlinearity at about 571° C., indicating that at about 571° C. thereaction between Al and Ni proceeds to form an Al₃Ni₂ alloy. The phasechange is also evidenced in the XRD profile. As shown in FIG. 6B, phasesof Al and Ni remains at a temperature around 571° C. Thus, byintroducing a barrier layer stack between the Al layer and Ni layer, thereaction temperature for Al and Ni couples can be increased to 571° C.

FIG. 6C shows an sheet resistance of the second exemplary reactivematerial stack as a function of heating time when the second exemplaryreactive material stack is held isothermally at 400° C. for 4 h. Asshown in FIG. 6C, there is no increase in sheet resistance as timepasses, indicating that the reaction between Al and Ni does not occur at400° C.

FIG. 7 is a graph summarizing ignition temperatures of reactive materialstacks employing various barrier layers of the present application. Eachreactive material stack has a structure represent by 10 nm Ni/X/20 nmAl/SiO₂, and X represents a barrier layer of the present application. Asshown in FIG. 7, by varying the composition and thickness of the barrierlayers, the reaction temperature of the reactive material stacksincluding Al and Ni reactive material pairs can be tailored to be from260° C. to 571° C.

In the present application, by introducing a barrier layer betweenlayers of the first reactive material and second reactive material, theignition temperature of resulting reactive material stacks can tuned.The reactive material stacks thus formed are benign during the chipfabrication and chip operation, but can be ignited when a triggeringevent occurs at a desired time. Further, by varying composition andthickness of the barrier layer of the present application, the ignitiontemperatures of the reactive material stacks can be tuned. The designflexibility can be greatly improved.

While the application has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Each of the embodiments described herein canbe implemented individually or in combination with any other embodimentunless expressly stated otherwise or clearly incompatible. Accordingly,the application is intended to encompass all such alternatives,modifications and variations which fall within the scope and spirit ofthe application and the following claims.

1. A reactive material stack comprising: alternating layers of a firstreactive material and a second reactive material; and a barrier layerlocated between the layers of the first reactive material and the secondreactive material, wherein the barrier layer includes a materialselected from the group consisting of vanadium oxide, hafnium nitrideand niobium nitride, or a stack of, from bottom to top, Ta/Ta_(x)O_(y),Al_(x)O_(y)/Ta/Ta_(x)O_(y) or Al_(x)O_(y)/Ta/Ta_(x)O_(y)/Ta/Ta_(x)O_(y).2. The reactive material stack of claim 1, wherein the barrier layer islocated between each layer of the first reactive material and the secondreactive material.
 3. The reactive material stack of claim 1, whereinthe barrier layer is located between each pair of the layers of thefirst reactive material and the second reactive material. 4.-6.(canceled)
 7. The reactive material stack of claim 1, wherein the firstreactive material and the second reactive material are selected from thegroup consisting of Ni/Al, Al/Pd, Cu/Pd, Nb/Si and Ti/Al.
 8. Thereactive material stack of claim 1, wherein each of the layers of thefirst reactive material and the second reactive material has a thicknessfrom 1 nm to 200 nm, and the barrier layer has a thickness from 1 nm to20 nm.
 9. The reactive material stack of claim 1, wherein the reactivematerial stack has a total thickness from 0.5 μm to 10 μm. 10.-20.(canceled)
 21. The reactive material stack of claim 1, wherein the firstreactive material and the second reactive material reacts with eachother, but not with the barrier layer, to form an alloy or a compositeconsisting of the first reactive material and the second reactivematerial.